LOS ANGELES — Two days before Justin Bieber’s 18th birthday he was, as usual, working. He walked into the main control room at Record Plant, a recording studio here, his blue-and-pink high-top Balenciaga sneakers unlaced, nodding his head to a sinuous beat blaring over the speakers.

On this Tuesday night in February Mr. Bieber was a few weeks into recording his second full-length album, “Believe” (RBMG/Island). The studio was crowded with songwriters trying to turn that beat into a hit; with Mr. Bieber’s relatives, in town for his birthday; with a security guard; with various assistants.

Then there was Kuk Harrell, the only person not openly vying for Mr. Bieber’s attention, who moved through the scrum quietly, every so often checking settings on a computer.

Mr. Harrell is Mr. Bieber’s vocal producer, filling a many-layered and amorphous job: part vocal coach, part cheerleader, part sound engineer, part therapist. At this studio he’s a star, too: the nameplate on a kick scooter at the front desk just says, “KUK.”

Pop music’s universe of celebrities has widened in recent years to include producers and songwriters; they’re as crucial to what you hear on the radio as the stars, and increasingly known to the public. But there are deeper levels of highly specialized talent, just as integral, that often go unrecognized.

Mr. Harrell, 47, is one of those figures, shaping the sound of radio from the shadows. His client roster also includes Jennifer Lopez and Rihanna, and his job is to make sure that the star’s vocal is as powerful and flawless as it can be.

That happens in parts. In the studio, rarely, if ever, does a star sing a song the whole way through. Instead Mr. Harrell builds a gleaming whole from granular bits. A singer working with Mr. Harrell covers a few bars — a line or two, maybe four — over and over, with different emphases and inflections, until Mr. Harrell hears what he wants. The process repeats for each section. Only later, after the singer is gone, does Mr. Harrell stitch the best pieces together, Frankenstein-like, into the song you hear.

On this night Mr. Bieber retired to a smaller studio in the back of the building to work on “Sunday Morning,” an aching midtempo ballad, warming up by singing long stretches of the song. But once Mr. Harrell sat down at the computer, they broke “Sunday Morning” down into small vocal bites, with Mr. Bieber sometimes echoing what Mr. Harrell sang to him moments before. All the while Mr. Harrell’s eyes remained fixed on the computer screen, where each new take was represented in ProTools, the production software, by a jagged line, like heart-rate tracings on an EKG, inside a brightly colored rectangle. The data were piling up.

Stitching the Best Bits

Vocal production can be a tedious task, requiring patience, indulgence and discipline. Mr. Harrell’s work begins long before his client shows up, as he determines what kinds of microphone, preamplifiers and compressor will be best. When the singer is ready, he kicks into high gear.

“I’ll take it in chunks,” Mr. Harrell explained, sitting in his all-white apartment the day before the Bieber recording session. “If they sang it amazing, I’ll get the first chunk and go, ‘Oh that was beautiful.’ Boom. I’ll drag that up.”

The process then repeats. “So I’ll begin to search through all the playlists I have of that ‘moment’ and go, ‘Ah, there it is,’ then I’ll drag that moment up into the comp” — the compiled vocal — “and so on. All the way down, until I get to the end of the record and have a complete final comp.”

In the studio he’s enthusiastic but calm, direct and not sycophantic. Lean and slight, he’s got a high hairline and long curtain of hair that recalls the comedian Katt Williams. Most of the time he’s at the computer, a cap often pulled low over his eyes, his back upright and rigid. Even though he’s seated, he appears to be on tiptoes.

After an hour or two of takes the singer’s work is done. Mr. Harrell then “tunes” the compiled vocal, making sure the pitch is precise, and “grooves” it, matching it precisely to the rhythm of the backing track.

Though he’s reliant upon technology, Mr. Harrell insists that most of his work is navigating personalities, getting stars to trust him when they’re in the recording booth, at their most vulnerable. “It’s never, ‘Man, you screwed up,’ ” he said. “I can tell Jennifer she’s not singing it the right way without telling her that she’s not singing it the right way: ‘Give it a sexy vibe like you’re singing in the shower,’ or ‘Sing it like no one else is in the room.’ ”

Ms. Lopez said that Mr. Harrell “can find your strengths, and he can pull those out,” adding, “Everything is always done in an encouraging sort of way. One of his favorite lines — I don’t know if he uses it with everybody, but he uses it with me — is: ‘That’s a superstar performance right there! That’s it!’ And it just makes you feel so great about what you’re doing.”

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Kuk Harrell, a vocal producer, has a client roster that includes Justin Bieber, Jennifer Lopez and Rihanna.Credit
Chad Batka for The New York Times

For each of his regular artists Mr. Harrell has a bank of such phrases. “I have this term that I use with Jennifer all the time,” he said. “I go, ‘Put that cry in it, let me hear that cry,’ and she knows what that means.” With Rihanna he’ll push her with “There she is! She just showed up!”

The final product then, is an amalgam: the songwriter’s original lyric and melody, the singer’s particular tone and approach, and Mr. Harrell’s instinct and ear for how the artist can sound.

“He knows where my voice can sit and what notes I can hit,” Mr. Bieber said. “Sometimes I wouldn’t try for it ’cause I didn’t think I could hit it, and then if he goes, ‘Just go for it Justin,’ I go for it.”

For Mr. Bieber, whom Mr. Harrell has been working with since his first professional recordings four years ago, Mr. Harrell has “become kind of an uncle,” Mr. Bieber said.

But Mr. Harrell noted, “If I’m not careful, I can let that turn into, ‘We’re buddies, we hang out,’ and I can’t press him.”

“With Justin I always go, ‘Give me J. B.’ And he goes, ‘What’s J. B.?’ And I’ll go, ‘You know, J. B. is that.’ ” Here Mr. Harrell does a credible impression of Mr. Bieber’s falsetto. “When I show him that, he goes, ‘Oh, I got it.’ They just needed me to show them exactly what it was.”

Mr. Harrell’s clients also look to him for guidance on what they might not be able to pull off on a certain day. “Rihanna will walk in, and if we have a bunch of songs to do, she’ll say, ‘What do you want to do, Kuk?’ ” he said. “She understands that I’ve watched her. I should know as her vocal producer, if her voice is pretty tired, let’s record all the stuff that’s in a lower register. Let’s not try to get high notes because I know we won’t get there.”

When superstars work with Mr. Harrell, they aren’t running to the machines and away from their own voices. Quite the opposite: they’re trying to ensure that they sound as engaged and alive as possible. Paradoxical as it seems, working with newfangled technology and old-fashioned pep talks Mr. Harrell makes singers sound even more like themselves.

“We want to enhance the artist’s authenticity,” said Chris Hicks, who was until recently executive vice president at Island Def Jam, home to Mr. Bieber, Ms. Lopez and Rihanna. “You buy a Bieber or Rihanna because you believe in them, and this is part of that.”

And so it falls to Mr. Harrell not just to elicit sterling vocal performances, but also to preserve and highlight what’s distinctive about each voice: Ms. Lopez’s blend of husk and flirt, Rihanna’s petulant purr, Mr. Bieber’s sweet coo.

Mr. Harrell, whose given name is Thaddis, was born in 1964 into a musical Chicago family. His mother, Vivian Haywood, and her two sisters were “the go-to jingle singers in Chicago,” he said. Mr. Harrell was a studio rat, watching and learning, while drumming in the school band, singing in the church choir and ordering Heart, Spyro Gyra and Van Halen albums from a mail-order record club.

In the early 1990s he and several family members — including one of his cousins, the future songwriter Tricky Stewart (Mariah Carey’s “Touch My Body,” Britney Spears’s “Me Against the Music”) — moved to Los Angeles to give the record business a go. But after a few years with limited success Mr. Harrell became disillusioned.

A neighbor convinced him to visit the Christian Assembly Foursquare Church in Eagle Rock, near Pasadena, where Mr. Harrell was first exposed to the world of worship music, the ecstatic rock-driven prayer sessions that were just then beginning to become common in larger churches, a complement to gospel’s role in the black church. Watching the worship leader, Tommy Walker, Mr. Harrell recalled, “I remember thinking, ‘This sounds like Toto,’ ” the polyglot soft-rock band (“Africa”) popular in the early 1980s. “ ‘I’ve never seen this before in church.’ ”

Soon he’d abandoned the music industry altogether to become the church’s children’s worship leader. “I heard it clear as day,” he said. “God speaking in my heart and going: ‘Look at what you’re doing. There will never ever be anything more important than what you’re doing at this moment.’ ”

A few years later he began working with the music side of the Promise Keepers, the mid-1990s Christian men’s movement. His first gold album came for some production work he did on the 1996 Promise Keepers album, “Break Down the Walls” (Maranatha). Eventually he and Mr. Stewart joined up again.

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Mr. Harrell, center, worked with Mr. Bieber, right, on the pop star's recently released second album, “Believe.”Credit
Stephanie Diani for The New York Times

“It turned into, ‘O.K., you just produce vocals all day, give the files back to me,’ Jimmy Jam and Terry Lewis, tight production,” Mr. Harrell said of those days, referring to the producers who gave Janet Jackson her early sonic identity. Mr. Harrell’s years as a worship leader, he said, gave him the people skills and humility to work with superstars.

He and Mr. Stewart began getting high-profile work. They helped write “Umbrella,” one of the signature pop songs of the last decade, for Rihanna, and for which Mr. Harrell won one of his four Grammys.

Working with Mr. Stewart, Mr. Harrell began to refine his talent for producing vocals to the level where his biggest clients won’t record without him.

For a singer (and his or her label), having a vocal producer is still an indulgence, available only to the heavily budgeted and fastidious. Having the certainty of Mr. Harrell’s ear comes with a price: several thousand dollars per song and, more significant, a cut of the royalties.

Those can add up. “Believe,” Mr. Bieber’s album which was released last week, had the biggest first week of any album this year, selling 374,000 copies, according to Nielsen SoundScan. And its first single, “Boyfriend,” has already sold more than 2.5 million copies.

“Doing what I do you can make over a million a year,” Mr. Harrell said in an e-mail.

He has also expanded beyond vocal production. On the Bieber album he was the overall producer, supervising all of the songs at every stage of production. Interscope has given him his own imprint, Suga Wuga Entertainment, to which he’s signed a sister pop trio, Calvillo, and a rock band, Savannah Van Band. And for his primary clients he’ll often join them outside the studio to work on other projects: live shows, TV appearances and more. (He supervised the audio on last year’s 3-D concert movie “Justin Bieber: Never Say Never.”)

The pace can be relentless. During one stretch last year Mr. Harrell was working with Rihanna in London, while wrapping up Mr. Bieber’s Christmas album in his downtime, flying back to the United States a couple of times to work with Ms. Lopez. In between the Bieber sessions in February, Mr. Harrell squeezed in a night working on a song for Ciara, and when Mr. Bieber took a few days off around his birthday, Mr. Harrell got a call to work on the debut album by Melanie Amaro, the “X Factor” winner. And this month he shuttled around Latin America, traveling to Panama, Venezuela, Chile, Argentina and Brazil as he accompanied Ms. Lopez on her world tour. So far this year — in which he’s also been working with Cher, Celine Dion and Keyshia Cole — he’s taken off just 10 days.

In His Element

The night after the attempt at “Sunday Morning” (which would not end up making the album) Mr. Harrell was back at Record Plant with Mr. Bieber. Again the singer was distracted; at midnight it would be his birthday. “I’m gonna get my Black Card,” he sang, “Order it tomorrow-oh-oh.”

He was working on “Believe,” the title track. An elegant, yearning love song about loyalty and trust, “Believe” could be about a family member, a girlfriend, or really, millions of fans. Mr. Bieber went into the booth to work on the swelling portion of the song.

“Just look at me nowwww,” Mr. Bieber sang.

Mr. Harrell said, “Oooooo, put that passion on it,” his face scrunched slightly as if a light odor had just hit his nose, a sign of approbation. “Try to catch that mmmmmmm.”

Mr. Bieber took the note: “Just look at mmmeee now.”

Mr. Bieber soon shifted to work on the song’s soaring hook, squeezing power from his light voice: “I don’t know how I got here/I knew it wouldn’t be easy/Doesn’t matter how many times I got knocked on the floor.”

Mr. Harrell shouted: “I love it. I love that soft tone too.”

Mr. Bieber called back, “Do it again?”

“Absolutely,” Mr. Harrell said. “You killin’ it. I just need to understand you just a little bit more.”

Mr. Bieber tried again. “I don’t know how I got heeeerrrre/ I knew it wouldn’t be eeeeasyyyy,” he sang, hitting the last word of each line with more power and tang.

“Wooooooo!” Mr. Harrell exulted. “That’s incredible.”

Mr. Bieber came out of the booth to hear his vocals played back. “I’m so excited for my birthday, I can hardly even concentrate,” he said.